Tafel Slopes and Potential Dependence of Coverage by Intermediates

Tafel Slopes and Potential Dependence of Coverage by Intermediates 

We have indicated above that for a simple electron-transfer reaction, not involving a chemisorbed intermediate, or for such a step in a more complex process where the coverage, 9, by intermediates is small (say, 1%, when the discharge step producing the intermediate is rate controlling) the Tafel slope dVjd ni is simply 

Br0nsted factor for the effect of electrode potential on the Fermi level of electrons, causing change of AG with V (see p. 6). 

When a prior discharge step producing the adsorbed intermediate, for example, H in the HER, can be considered in quasi-equilibrium when the desorption step is rate controlling (an experimentally encountered situation at a number of metals), 6 is expressed by an electrochemical Langmuir-type adsorption relation  

It is seen, however, that for strong polarization conditions, KC,e p(VFIRT) can become comparable with or much greater than 1, so that 0 - 1, and then ft = RT/pF, so that distinction of rate control by the value of ft can no longer be made. However, under these conditions, 0 then being large, some other methods (e.g., potential-relaxation and/or impedance measurements and analysis) are then applicable (see Section VI). 

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